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Jun 19, 2018 - Wales 2052, Australia. •S Supporting Information. ABSTRACT: We devised a multipronged approach to validate the mechanism of oxalate C...
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Multipronged validation of oxalate C-C bond cleavage driven by Au-TiO2 interfacial charge transfer using operando DRIFTS Tze Hao Tan, Roong Jien Wong, Jason Scott, Yun Hau Ng, Robert A. Taylor, Kondo-Francois Aguey-Zinsou, and Rose Amal ACS Catal., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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Multipronged validation of oxalate C-C bond cleavage driven by Au-TiO2 interfacial charge transfer using operando DRIFTS Tze Hao Tan1, Roong Jien Wong1, Jason Scott*,1, Yun Hau Ng1, Robert A. Taylor2, Kondo-Francois Aguey-Zinsou1, Rose Amal*,1. 1.

School of Chemical Engineering, The University of New South Wales (UNSW), Kensington, New South Wales 2052, Australia

2.

School of Mechanical and Manufacturing Engineering, The University of New South Wales (UNSW), Kensington, New South Wales 2052, Australia

ABSTRACT: We devised a multipronged approach to validate the mechanism of oxalate C-C bond cleavage and the role of Au/TiO2 interfacial interaction via (i) selective photo-driven electronic transfer and (ii) tempering of the electronic interaction by alloying Au with Cu, which serves as an electron sink. This approach allows us to speed up, slow down, impede and bypass the interfacial electronic transfer. The multipronged approach led to a singular conclusion: Au-TiO2 electronic interaction played a critical role in C-C bond cleavage by fixating bidentate oxalate species at the Au-TiO2 interfacial perimeter with parallel C-C bonds to the TiO2 surface.

KEYWORDS: Gold, titanium dioxide, DRIFTS, interfacial interaction, C-C bond cleavage

The use of supported gold (Au) nanoparticles as heterogeneous catalysts and visible light active photocatalysts has generated a lot of research interest with new findings published on a regular basis. Au-based catalysts have been used extensively in green chemistry and environmental catalysis.1–4 Omri recently reported on Au-driven photoactivation for the selective oxidation of sugars into corresponding aldonates, which may be employed as a substitute to petroleum based precursors.5 Au/TiO2 has also been demonstrated to be excellent for cleaving C-C bonds in organics, a step critical for selective organic synthesis and degrading volatile organic carbons.6–10 In earlier work, we used diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) to show that Au/TiO2

cleaved C-C bonds in ethanol via the formation of oxalate species at the Au-TiO2 interfacial perimeter, facilitated by interfacial electronic interaction between Au and TiO2.9,10 Prior to that, the C-C bond cleavage of alcohol species was only hinted at through the formation of single-carbon products such as carbon dioxide and formate intermediate species.6,7,11,12 Irrespective of this, the exact cleavage mechanism remains poorly understood, particularly as it is obscured by the overlapping C-O infrared vibration contributions from various intermediate species in the CO fingerprint region. Herein, we validate the mechanism (Scheme 1) through direct observation of cleaving of the adsorbed oxalate species on the of Au/TiO2 surface via

Scheme 1 Multipronged approach based on (i) selective photo-excitation, and (ii) modification of interfacial electronic interaction, to validate the role of Au-TiO2 interfacial interaction in C-C bond cleavage: (a) monodentate adsorption of monodeprotonated oxalic acid, (b) deprotonation of oxalic acid to form bidendate oxalate with parallel C-C bond, (c) further formation of bidentate bond leading to cleavage of the C-C bond. Electronic pathway at the Au-TiO2 interfacial perimeter is illustrated by the red arrows.

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Figure 1 Transient in-operando DRIFTS spectra spanning 1850 - 1150 cm in ascending time steps (0 - 120 min, 5 min intero vals) for (a) Au/TiO2 and (b) neat TiO2 during oxalic acid oxidation in air at 100 C without illumination (i.e. in the dark). Air flow rate = 20 mL/min, reaction performed at ambient pressure.

operando DRIFTS. An extensive database of infrared spectroscopy on the adsorption and ultraviolet (UV) photo-degradation of oxalic acid on TiO2 can be found in the works of McQuillan et al.,13,14 Hug et al.15–17 and Bahnemann et al.16,18–20 Organics mainly adsorb on the TiO2 surface of Au/TiO2; thus, the compiled infrared spectra on TiO2 adsorbed oxalate species are valuable for the interpretation of DRIFTS data collected herein.9,21 Bahnemann et al. went one step further, using theoretical calculations to simulate the coordination chemistry of adsorbed oxalic acid on both anatase and rutile.19,20 These works enable definition of the coordination chemistry of adsorbed oxalic acid that is being cleaved during the catalytic/photocatalytic oxidation process. Nonetheless, it is notable that infrared vibrations shift with changes in coordination chemistry, environment, temperature, and concentration.22 Previous studies demonstrated that the electronic pathways of Au/TiO2 can be tuned by: (i) selective photodriven electronic transfer under visible (plasmonmediated electron transfer) and ultraviolet (bandgap photoexcitation) illumination; and (ii) manipulation of the Au-TiO2 interfacial interaction by alloying Au with metals of different electron affinity – e.g. copper (Cu) and platinum (Pt).9,10,21 On utilising this multipronged approach, we are able to affirm the role of Au-TiO2 interfacial interaction in cleavage of the oxalate C-C bond. Methods. To enable the study of adsorbed oxalic acid oxidation in air, oxalic acid was first adsorbed onto the surface of synthesised catalysts via solution mixing under constant nitrogen purge and vacuum dried overnight to minimise oxidation of the adsorbed species. The dried catalyst was then transferred to the DRIFTS cell and heated to 100 oC under a nitrogen gas flow (10 mL/min). During the DRIFTS analyses, the inlet gas was changed to zero air (20% oxygen in nitrogen balance) and monitored

for 2 hr. Quantitative comparisons of intermediate species were enabled through post-treatment of the DRIFTS spectra to remove discrepancies arising from varying detection sensitivity, signal noise, and background interference from TiO2. The corrected spectra were curve fitted and integrated in Fityk 0.9.8 based on the LevenbergMarquardt method, assuming that the peak shapes comprise of a mixture of Lorentzian and Gaussian functions (Figure S1).23 Au/TiO2 and corresponding bimetallic variants were synthesised using the deposition-precipitation method and characterised as described elsewhere.21,24 The detailed experimental setup (Scheme S1) and characteristic information on the synthesised catalysts (Figure S2-4) are available in the Supporting Information. Approach I: Effects of Selective Photo-excitation. Figure 1 shows the transient corrected DRIFTS spectra of Au/TiO2 and the neat TiO2 during oxalic acid oxidation at 100 oC in the dark. At time = 0 min, C-O vibration peaks corresponding to carboxylates of adsorbed oxalic acid on the surface of TiO2 can be observed at 1736 cm-1, 1680 cm-1, and 1630 cm-1 on both Au/TiO2 and neat TiO2. The formation of three peaks in the region of 1500 – 1800 cm-1 were consistent with the works of Hug et al.15–17 and Bahnemann et al.16,18–20 Hug et al. ascribed the observed vibrations at above 1700 cm-1 and 1680 cm-1 to the symmetric and asymmetric combinations of two C=O double bonds, νs(C=O, Ox) and νas(C=O, Ox). Both Hug and Bahnemann assigned the peak at 1630 cm-1 to the asymmetrical O-C=O stretching vibrations, νas(O-C=O, Ox). Significant peak shifts (50 – 60 cm-1) were observed in the wavenumber region of < 1500 cm-1 compared to the literature database. Nonetheless, comparison between features of the observed vibration peaks suggested that they correspond to the same species. As such, vibration peaks in the range of 1355 – 1368 cm-1 (νs(C-O, Ox) + νs(C-C, Ox)) and 1207 – 1216 cm-1 (νas(C-O, Ox) + νas(C-C, Ox)), can be confidently ascribed to combinations of C-O and C-C stretching vi-

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brations, respectively.16,19,20 Based on molecular dynamics

Figure 2 Integrated peak areas of deconvoluted infrared vibration peaks derived from transient in-operando DRIFTS spectra o corresponding to Au/TiO2 during oxalic acid oxidation in air at 100 C under different illumination conditions: dark, visible and UV illumination (Figure S5). Peak areas plotted in ascending time steps (0 - 120 min, 5 min intervals). List of deconvo-1 luted peaks: (a) 1500-1800, (b) 1355-1368, (c) 1410 – 1447, (d) 1528-1532, (e) 1207-1216, and, (f) 1227-1237 cm .

simulations by Bahnemann et al., these vibration peaks can be ascribed to monodentate (Scheme 1a) and bidentate oxalate species with the C-C bonds parallel to the TiO2 surface (Scheme 1b).19,20 In addition, the shoulder located at 1227 - 1237 cm-1 can be assigned to a protonatedor strongly hydrogen bonded oxalate species on the

TiO2 surface. Inspection of the transient DRIFTS spectra of Au/TiO2 (Figure 1a) shows gradual decay of the peaks at 1500 – 1800 cm-1 with time. This is accompanied by immediate formation of new peaks at 1410 – 1447 cm-1 and later at 1528 – 1532 cm-1. The peaks at 1410 – 1447 cm-1 and 1528 – 1532 cm-1 are ascribed to the symmetric C-O

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stretching vibration of formate species, νs(C-O, HCOO) and the asymmetric C-O stretching vibration of carbonate species, νas(C-O, CO3), respectively.25–27 While residual formate species was observed on neat TiO2, the almost exclusive formation of formate and carbonate species confirmed cleaving of the oxalate C-C bond by Au/TiO2. The addition of hydrogen to the cleaved oxalate species, to form formate species, suggests that surface adsorbed water or the O-H group plays a role in cleaving the C-C bonds. Under visible illumination the decay rate of the vibration peaks at 1500 – 1800 cm-1 and 1355 – 1368 cm-1 was increased as indicated by the integrated peak area (Figure 2a, b). The enhancement in catalytic oxidation arises from the improved electron transfer under illumination at the Au-TiO2 interface as previously demonstrated (Scheme 2b).9,10,28,29 Nonetheless, the time taken for formate species to reach equilibrium was increased by 20 min compared to unilluminated Au/TiO2 (Figure 2c), which is caused by further oxidation of formate species to carbon dioxide and carbonate species (1528 – 1532 cm-1, Figure 2d) under visible light illumination. The rate of decay of vibration peaks 1500 – 1800 cm-1 and 1355 – 1368 cm-1 under UV illumination (Figure 2a, b) suggests an alternative oxidation pathway via direct bandgap photoexcitation over the whole TiO2 surface (Scheme 2a), which suppresses the formate species accumulation (Figure 2c).

at 1500 – 1800 cm-1 and 1355 – 1368 cm-1 (Figure 3a, b), a similar degradation rate of oxalate species compared to Au/TiO2 can be observed for AuPt/TiO2. In addition, an increased initial concentration of formate species (1410 – 1447 cm-1, Figure 3c) is observed on the AuPt/TiO2 surface

A separate trend was observed for the vibration peak at 1207 – 1216 cm-1 for the dark and illuminated Au/TiO2 catalysts (Figure 2e), where the peak area for dark Au/TiO2 increases gradually over time while the illuminated Au/TiO2 (UV and vis) decreases over time. This vibration peak is ascribed to bidentate oxalate species with C-C bonds parallel to the TiO2 surface (Scheme 1b).19,20 In a separate theoretical study, Sun et al. found that increased electron transfer to the adsorbed oxalate species promoted formation of double bidentate species on the TiO2 surface (Scheme 1c), which eventually led to cleaving of the C-C bond.30 Based on our findings and reported theoretical studies, we propose that the Au-TiO2 interfacial perimeter promotes cleaving of oxalate C-C bonds by promoting bidentate oxalate species formation with C-C bonds parallel to the TiO2 surface (Scheme 1b).30 This occurs via electron donation from the TiO2 surface to the oxalic species and donation of an electron from the oxalic species (oxidation) to the Au deposit, which is facilitated by the interfacial electronic transfer from Au to TiO2. Consequently, increased electron transfer from Au under visible illumination promotes formation of the double bidendate species (Scheme 1c) and cleaving of C-C bond, leading to the decrease in peak area at 1207 – 1216 cm-1 (Figure 2e). Approach II: Effect of Secondary Alloy Interface. Subsequently, we examined the role of electron transfer at the Au-TiO2 interfacial perimeter by modifying it with a secondary alloy interface. From XPS analyses of the monometallic and alloyed species (Figure S4), negligible Au 4f4/2 binding energy shifts were observed when Au was alloyed with Pt (Figure S4a). Based on the peak area trend

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Scheme 2 Proposed effects of selective photoexcitation (top) and tempering of electronic interactions (bottom) on the interfacial interaction of Au/TiO2: (a) UV illumination excites TiO2, mitigating the role of Au; (b) visible light illumination enhances Au-TiO2 charge transfer; (c) electronic transfer at the Au-TiO2 interface in the dark; (d) suppression of Au-TiO2 interaction by Pt ACS Paragon Plus Environment as an electron sink; and (e) complete impedance of Au-TiO2 interaction by Cu.

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(10 a.u.) compared to Au/TiO2 (8 a.u.). Examination of the

vibration

Figure 3 Integrated peak areas of deconvoluted infrared vibration peaks derived from transient in-operando DRIFTS spectra o corresponding to Au/TiO2, AuCu/TiO2, and AuPt/TiO2 during oxalic acid oxidation in air at 100 C (Figure S6). Peak areas plotted in ascending time steps (0 - 120 min, 5 min intervals). List of deconvoluted peaks: (a) 1500 – 1800, (b) 1355-1368, (c) -1 1410 – 1447, (d) 1528-1532, (e) 1207-1216, and, (f) 1227 – 1237 cm .

peak at 1207 – 1216 cm-1 (Figure 3e) shows that AuPt/TiO2 still actively promotes bidentate oxalate species formation with parallel C-C bonds. The slight reduction in catalytic activity for AuPt/TiO2 suggests that Pt may act as an electron sink, diverting some of the electrons from the AuTiO2 interface (Scheme 2d). Conversely, we observed a

strong interaction between Au and Cu, with corresponding positive (Au 4f7/2, +0.4 eV) and negative (Cu 2p3/2, -0.3 eV) binding energy shifts (Figure S4). This may result in reduction of the fermi level, Ef, of Au (Scheme 2e). Consequently, the presence of u in AuCu/TiO2 completely subdues the catalytic activity of Au-TiO2, creating a tran-

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sient DRIFTS profile similar to neat TiO2 (Figure 3a, b). Extremely slow formate species (1410 – 1447 cm-1) formation is observed over time, although no carbonate species (1528 – 1532 cm-1) formation was observed (Figure 3d). A small vibration peak located at 1237 cm-1, which slowly decays over time, can be observed on AuCu/TiO2 (Figure 3f). We believe that this corresponds to a slow migration of residual protonated oxalate species from Cu sites to the TiO2 surface. The Cu in AuCu/TiO2 was previously shown to also function as active sites for organic adsorption.21 However, this migration was inconsequential in relation to oxalate C-C cleavage as monodentate oxalate species are already available in abundance Figure 3b). Overall, we are confident that cleaving of the oxalate C-C bonds is impeded by the selective electronic interaction between Au and Cu (Scheme 2e). Herein, we have demonstrated, via selective photoexcitation (different excitation light sources) and the addition of a secondary alloy interface, a multipronged approach to validate the role of Au-TiO2 interfacial interaction in oxalate C-C bond cleavage. In this study, transient DRIFTS spectra of adsorbed oxalic acid showed that the cleaving of oxalate species: (i) led to formate species formation which were later converted to carbonate species; (ii) occurred on the surface of Au/TiO2 but not TiO2, suggesting that the Au-TiO2 interfacial perimeter played a role in the cleaving process; and (iii) proceeded through bidentate oxalate species formation with C-C bonds parallel to the TiO2 surface. The multipronged approach leads to a singular conclusion: that the Au-TiO2 interfacial electronic interaction is critical for cleaving of the C-C bond. Under visible illumination, improved plasmon-mediated charge transfer increased the cleaving rate of oxalate C-C bonds, resulting in faster degradation of adsorbed oxalic species. In contrast, impedance of the Au-TiO2 interfacial interaction in the presence of Cu facilitated the stable adsorption of oxalate species on the TiO2 surface in air. Elucidation of the C-C bond cleavage by Au/TiO2 provides insight into the role of metal-oxide interfacial perimeter in catalytic reactions. Additionally, in having direct control of the Au-TiO2 interfacial electronic transfer process at our fingertips we are able to speed up, slow down, and even halt the C-C bond cleavage process. This implies that we may be able to control the kinetics and selectivities of metal-oxide support interfacial interaction dependent reactions.

Rose Amal: 0000-0001-9561-4918 Author Contributions The manuscript was written through contributions of all authors. T. H. Tan devised and conducted all DRIFTS analyses and writing. R. J. Wong assisted with catalysts preparation and characterisations. All authors have given approval to the final version of the manuscript.

Funding Sources The work was supported by the Australian Research Council (ARC) under the Laureate Fellowship Scheme – FL140100081.

ACKNOWLEDGMENT The authors would also like to acknowledge the use of facilities within the UNSW Mark Wainwright Analytical Centre and UoW Electron Microscopy Centre.

REFERENCES (1)

Corma, A.; Garcia, H. Supported Gold Nanoparticles as Catalysts for Organic Reactions. Chem. Soc. Rev. 2008, 37, 2096–2126.

(2)

Primo, A.; Corma, A.; García, H. Titania Supported Gold Nanoparticles as Photocatalyst. Phys. Chem. Chem. Phys. 2011, 13, 886–910.

(3)

Stratakis, M.; Garcia, H. Catalysis by Supported Gold Nanoparticles: Beyond Aerobic Oxidative Processes. Chem. Rev. 2012, 112, 4469–4506.

(4)

Shahzad, S. A.; Sajid, M. A.; Khan, Z. A.; Canseco-Gonzalez, D. Gold Catalysis in Organic Transformations: A Review. Synth. Commun. 2017, 47, 735–755.

(5)

Omri, M.; Sauvage, F.; Busby, Y.; Becuwe, M.; Pourceau, G.; Wadouachi, A. Gold Catalysis and Photoactivation: A Fast and Selective Procedure for the Oxidation of Free Sugars. ACS Catal. 2018, 8, 1635–1639.

(6)

Holz, M. C.; Tölle, K.; Muhler, M. Gas-Phase Oxidation of Ethanol over Au/TiO2 Catalysts to Probe Metal–support Interactions. Catal. Sci. Technol. 2014, 4, 3495–3504.

(7)

Holz, M. C.; Kähler, K.; Tölle, K.; van Veen, A. C.; Muhler, M. Gas-Phase Oxidation of 2Propanol over Au/TiO2 Catalysts to Probe Metal-Support Interactions. Phys. Status Solidi 2013, 250, 1094–1106.

(8)

Andreeva, D.; Tabakova, T.; Ilieva, L.; Naydenov, A.; Mehanjiev, D.; Abrashev, M. V. Nanosize Gold Catalysts Promoted by Vanadium Oxide Supported on Titania and Zirconia for Complete Benzene Oxidation. Appl. Catal. A Gen. 2001, 209, 291–300.

(9)

Tan, T. H.; Scott, J.; Ng, Y. H.; Taylor, R. A.; Aguey-Zinsou, K.-F.; Amal, R. C–C Cleavage by Au/TiO2 during Ethanol Oxidation: Understanding Bandgap Photoexcitation and

ASSOCIATED CONTENT Supporting Information. Detailed experimental methods and characteristic information of synthesised catalysts are available. In addition, corrected transient DRIFTS spectra of Au/TiO2 under visible and UV illumination, AuCu/TiO2 and AuPt/TiO2 are available.

AUTHOR INFORMATION Corresponding Author Email: [email protected], [email protected] ORCID:

Jason Scott: 0000-0003-2395-2058

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ACS Catalysis Plasmonically Mediated Charge Transfer via Quantitative in Situ DRIFTS. ACS Catal. 2016, 6, 8021–8029.

(10)

(11)

(12)

Tan, T. H.; Scott, J.; Ng, Y. H.; Taylor, R. A.; Aguey-Zinsou, K.-F.; Amal, R. Understanding Plasmon and Band Gap Photoexcitation Effects on the Thermal-Catalytic Oxidation of Ethanol by TiO2-Supported Gold. ACS Catal. 2016, 6, 1870–1879. Sobolev, V. I.; Koltunov, K. Y.; Simakova, O. A.; Leino, A.-R.; Murzin, D. Y. Low Temperature Gas-Phase Oxidation of Ethanol over Au/TiO2. Appl. Catal. A Gen. 2012, 433–434, 88–95. Sobolev, V. I.; Simakova, O. A.; Koltunov, K. Y. Generation of Reactive Oxygen Species on Au/TiO2 after Treatment with Hydrogen: Testing the Link to Ethanol Low-Temperature Oxidation. ChemCatChem 2011, 3, 1422–1425.

M. A.; Bahnemann, D. Adsorption of Oxalate on Anatase (100) and Rutile (110) Surfaces in Aqueous Systems: Experimental Results vs. Theoretical Predictions. Phys. Chem. Chem. Phys. 2009, 11, 1794–1808. (21)

Tan, T. H.; Scott, J. A.; Ng, Y. H.; Taylor, R. A.; Aguey-Zinsou, K.-F.; Amal, R. Plasmon Enhanced Selective Electronic Pathways in TiO2 Supported Atomically Ordered Bimetallic AuCu Alloys. J. Catal. 2017, 352, 638–648.

(22)

Alpert, N. L.; Keiser, W. E.; Szymanski, H. A. IR: Theory and Practice of Infrared Spectroscopy; Springer US, 2012.

(23)

Marcin, W. Fityk: A General-purpose Peak Fitting Program. J. Appl. Crystallogr. 2010, 43, 1126–1128.

(24)

Wong, R. J.; Scott, J.; Kappen, P.; Low, G. K. C.; Hart, J. N.; Amal, R. Enhancing Bimetallic Synergy with Light: The Effect of UV Light PreTreatment on Catalytic Oxygen Activation by Bimetallic Au-Pt Nanoparticles on a TiO2 Support. Catal. Sci. Technol. 2017, 7, 4792–4805.

(13)

Young, A. G.; McQuillan, A. J. Adsorption/Desorption Kinetics from ATR-IR Spectroscopy. Aqueous Oxalic Acid on Anatase TiO2. Langmuir 2009, 25, 3538–3548.

(14)

Ekström, G. N.; McQuillan, A. J. In Situ Infrared Spectroscopy of Glyoxylic Acid Adsorption and Photocatalysis on TiO2 in Aqueous Solution. J. Phys. Chem. B 1999, 103, 10562–10565.

(25)

Chuang, C.-C.; Wu, W.-C.; Huang, M.-C.; Huang, I.-C.; Lin, J.-L. FTIR Study of Adsorption and Reactions of Methyl Formate on Powdered TiO2. J. Catal. 1999, 185, 423–434.

(15)

Borer, P.; Hug, S. J. Photo-Redox Reactions of Dicarboxylates and α-Hydroxydicarboxylates at the Surface of Fe(III)(Hydr)Oxides Followed with in Situ ATR-FTIR Spectroscopy. J. Colloid Interface Sci. 2014, 416, 44–53.

(26)

Mudalige, K.; Trenary, M. Identification of Formate from Methanol Oxidation on Cu(1 0 0) with Infrared Spectroscopy. Surf. Sci. 2002, 504, 208–214.

(27)

(16)

Hug, S. J.; Bahnemann, D. Infrared Spectra of Oxalate, Malonate and Succinate Adsorbed on the Aqueous Surface of Rutile, Anatase and Lepidocrocite Measured with in Situ ATR-FTIR. J. Electron Spectros. Relat. Phenomena 2006, 150, 208–219.

Liao, L.-F.; Wu, W.-C.; Chen, C.-Y.; Lin, J.-L. Photooxidation of Formic Acid vs Formate and Ethanol vs Ethoxy on TiO2 and Effect of Adsorbed Water on the Rates of Formate and Formic Acid Photooxidation. J. Phys. Chem. B 2001, 105, 7678–7685.

(28)

(17)

Hug, S. J.; Sulzberger, B. In Situ Fourier Transform Infrared Spectroscopic Evidence for the Formation of Several Different Surface Complexes of Oxalate on TiO2 in the Aqueous Phase. Langmuir 1994, 10, 3587–3597.

Tian, Y.; Tatsuma, T. Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632–7637.

(29)

(18)

Mendive, C. B.; Bahnemann, D. W.; Blesa, M. A. Microscopic Characterization of the Photocatalytic Oxidation of Oxalic Acid Adsorbed onto TiO2 by FTIR-ATR. Catal. Today 2005, 101, 237–244.

Sakai, N.; Fujiwara, Y.; Takahashi, Y.; Tatsuma, T. Plasmon-Resonance-Based Generation of Cathodic Photocurrent at Electrodeposited Gold Nanoparticles Coated with TiO2 Films. ChemPhysChem 2009, 10, 766–769.

(30)

(19)

Mendive, C. B.; Bredow, T.; Feldhoff, A.; Blesa, M.; Bahnemann, D. Adsorption of Oxalate on Rutile Particles in Aqueous Solutions: A Spectroscopic, Electron-Microscopic and Theoretical Study. Phys. Chem. Chem. Phys. 2008, 10, 1960–1974.

Sun, T.; Wang, Y.; Zhang, H.; Liu, P.; Zhao, H. Adsorption and Oxidation of Oxalic Acid on Anatase TiO2 (001) Surface: A Density Functional Theory Study. J. Colloid Interface Sci. 2015, 454, 180–186.

(20)

Mendive, C. B.; Bredow, T.; Feldhoff, A.; Blesa,

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Figure 1 Transient in-operando DRIFTS spectra spanning 1850 - 1150 cm-1 in ascending time steps (0 - 120 min, 5 min intervals) for (a) Au/TiO2 and (b) neat TiO2 during oxalic acid oxidation in air at 100 oC without illumination (i.e. in the dark). Air flow rate = 20 mL/min, reaction performed at ambient pressure. 181x72mm (150 x 150 DPI)

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Figure 2 Integrated peak areas of deconvoluted infrared vibration peaks derived from transient in-operando DRIFTS spectra corre-sponding to Au/TiO2 during oxalic acid oxidation in air at 100 oC under different illumination conditions: dark, visible and UV illumi-nation (Figure S5). Peak areas plotted in ascending time steps (0 - 120 min, 5 min intervals). List of deconvoluted peaks: (a) 1500-1800, (b) 1355-1368, (c) 1410 – 1447, (d) 1528-1532, (e) 1207-1216, and, (f) 1227-1237 cm-1. 185x217mm (150 x 150 DPI)

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Figure 3 Integrated peak areas of deconvoluted infrared vibration peaks derived from transient in-operando DRIFTS spectra corre-sponding to Au/TiO2, AuCu/TiO2, and AuPt/TiO2 during oxalic acid oxidation in air at 100 oC (Figure S6). Peak areas plotted in as-cending time steps (0 - 120 min, 5 min intervals). List of deconvoluted peaks: (a) 1500 – 1800, (b) 1355-1368, (c) 1410 – 1447, (d) 1528-1532, (e) 1207-1216, and, (f) 1227 – 1237 cm-1. 185x217mm (150 x 150 DPI)

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Scheme 1 Multipronged approach based on (i) selective photo-excitation, and (ii) modification of interfacial electronic interaction, to validate the role of Au-TiO2 interfacial interaction in C-C bond cleavage: (a) monodentate adsorption of monodeprotonated oxalic acid, (b) deprotonation of oxalic acid to form bidendate oxalate with parallel C-C bond, (c) further formation of bidentate bond leading to cleavage of the C-C bond. Electronic pathway at the Au-TiO2 interfacial perimeter is illustrated by the red arrows. 241x123mm (150 x 150 DPI)

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ACS Catalysis

Scheme 2 Proposed effects of selective photo-excitation (top) and tempering of electronic interactions (bottom) on the interfacial interaction of Au/TiO2: (a) UV illumination excites TiO2, mitigating the role of Au; (b) visible light illumination enhances Au-TiO2 charge transfer; (c) electronic transfer at the Au-TiO2 interface in the dark; (d) suppression of Au-TiO2 interaction by Pt as an electron sink; and (e) complete impedance of Au-TiO2 interaction by Cu. 90x275mm (150 x 150 DPI)

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TOC Graphic 151x85mm (150 x 150 DPI)

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